Details

Letter (in Dutch) of 4 November 1681; read in English at Royal Society, London, 9 November 1681; published in Dutch 1686, in Latin 1687. See Dobell 1932.
The description of one animalcule suggests that it was the flagellate protozoon Giardia lamblia, and this appears to be the first report of a protozoan parasite in humans. Leeuwenhoek found it, and various bacteria (including spirochetes) in his own diarrheic or slightly loose stools, but not in stools of ordinary firmness. He did not suggest a causative role for the microbes in the looseness of bowel movements -- and indeed the microbes may have played no role. On the available evidence, he would not have been justified in claiming causation, and there is no indication that he even suspected such a thing. Had Leeuwenhoek postulated a causative role, he would have reaped more attention from historians of medicine.
In the same letter, Leeuwenhoek described microbes, probably bacteria, in the feces of a hen and a pigeon.